<lark lines. Thus we can give the fishes of the litloral zone the general name of " marbleil 

 fishes ". They are sedentary, not very active fishes, having modest food requirements and 

 can scarcely be called voracious. Only during the mating season do they become active, this 

 being particularly true of the males. These take on a brilliant breeding dress, fight among 

 themselves and strive to " catch the eye " of dull and lazy females and then induce them to 

 spawn. After this, some species guard their eggs, only regaining their liberty when their 

 ofTspring have had a good start in life. This parental instinct is a particular feature of littoral 

 fi.^hes and is practically unknown in fishes of other ecological types. The eggs of the former 

 are attached. Some species build a nest, others carry the eggs themselves, others again are 

 viviparous. 



The freshivatcr type. — There are great biological diilerences among freshwater fishes, 

 differences that can be appreciated by comparing those living in mountain streams and leaping 

 in the torrents with those of the polluted, fetid waters of equatorial swamps, where respiration 

 becomes a problem. The freshwater type of fish comprises calm and peaceable plant-eaters, 

 sedentary and hardly voracious in habit, and ferocious predators, carnivores with a fearsome 

 appetite. They are markedly eurythermal, for river waters must follow the climatic changes 

 of the continents. Many species are camouflaged and take on the colours of the bottom or 

 the aquatic plants. Altogether they may be called " greenish fishes ". DifTerences between 

 the sexes and parental instincts are often well developed. 



These fishes reproduce by demersal eggs (attached eggs), some building a nest or guarding 

 the eggs or even carrying them about. Some species are viviparous. It will be evident that 

 in all these features the freshwater fishes are very like those of coastal waters. Yet they are 

 more adaptable with res])ect to changes of salinity, for a number of freshwater species move 

 down into estuaries or brackish lagoons. The anadromous forms are the extreme instances of 

 this euryhaline habit : indeed these are really freshwater fishes — for the fundamental habitat 

 of a living being must surely be defined by the medium in which it reproduces. 



Among the populations of stagnant swamps or of pools which dry up when it is very hot, 

 there is a tendency for some species to leave the water and become adapted to an aerial existence, 

 this being evident in special modifications of the gills or swim-bladder. This struggle to breathe 

 pure, life-giving oxygen also occurred in the far distant geological past when in Devonian times 

 primitive fishes, which crept about on paddle-shaped fins. left the streams to become amphibians 

 and began the conquest of the dry land. 



The nerilic type. — From the outer reaches of the littoral zone (where the water is some 

 20 fathoms deep and the great brown seaweeds and mud disappear) to the brink of the great 

 escarpment that sweeps down from about 110 fathoms to the abyss, extends the continental 

 shelf. This is a basal, submarine extension of the dry land. Where mountain chains border 

 the sea and around islands and volcanic archipelagos, it is quite narrow but it may reach an 

 immense size in continuation of low-lying coasts with gentle slopes. These enlargements of 

 the platform. " witnesses " of bygone subsidence, are covered by the shallower waters of conti- 

 nental seas. In the Atlantic Ocean near the European coasts, may be mentioned ; the Barents 

 Sea, the North Sea, the English Channel and the Irish Sea. Near the American coasts there 

 are the Laurentian Sea (Newfoundland Banks and banks oil Nova Scolia and New England) 

 and the Falklands Sea (ofT Argentina). In the Pacific and Indian Oceans : the Okhotsk Sea, 

 the bellow Sea, the Indonesian Sea, the North Australian Sea, the .Vndaman Sea and the Red 

 Sea; and near the Antarctic Continent, the Weddell Sea and the Ross Sea. 



Of the fishes living over the continental shelf, some are mid-water swimmers and belong 

 to coastal types, whilst oLliers live closer to the bottom. These coastal swimmers are for the 

 most part of standard form and belong to the great group of spiny-finned fishes, the dorsal 

 and anal fins of which are provided with pointed spines set in front of the soft rays. There 

 are groupers, sea breams and red mullets, while others, such as the gurnards, have an armoured 

 head. Near these coastal swiininns must also be placed coral-reef fishes with their powerful 



16 



